Lyme disease (LD), a tick-transmitted spirochetosis of man, is a major public health problem. A better understanding of the mechanisms of transmission of the causative agent, Borrelia burgdorferi, from wild animal reservoirs to man by ticks, or other routes, may elucidate ways to control LD. The objective of the proposed work is to examine the interactions between spirochetes and tick cells in vitro at the cellular, molecular and ultrastructural level, with the aim to develop methods for the in vitro propagation of vertebrate-infective spirochetes. Spirochetes cultured in isolation will be compared with those co- cultivated with tick cells (organ and cell cultures), as well as with spirochetes isolated from the vector tick, Ixodes dammini. The ability of spirochetes in continuous culture to reacquire infectivity in tick tissue cultures will be assessed. The retention of infectivity by new isolates (from tick or rodent tissue) co- cultivated with tick cells will also be evaluated. We will evaluate the retention of infectivity by fresh isolates (from ticks or vertebrate tissues) co-cultivated with tick cells. The influence of various culture parameters will be analyzed: feeder cell type, cell and spirochete density, medium composition, time of cultivation and passage frequency. Spirochete-tick cell interactions will be analyzed by quantitating the adherence of spirochetes to tick cells and the viability (infectivity/virulence) of cell-associated and cell-free spirochetes. Hamsters and mice will be compared for their suitability as monitoring systems for the infectivity and pathogenicity of the Lyme spirochete. Immunological (immunofluorescence, ELISA), microscopical (light, electron), biochemical (protein electrophoresis) and culture (endpoint titration, clonal growth) procedures will be used to detect, characterize and quantitate spirochetes in our cultures. The cultivation of spirochetes in tick cell cultures should reveal important information about the biology, infectivity and virulence of the Lyme spirochete for the vertebrate and have potential application in the etiology, epidemiology, immunology and pharmacology of LD.